(1) Field of the Invention
The present invention relates to a thin-film transistor array device, an organic EL display device, and a method of manufacturing the thin-film transistor array device.
(2) Description of the Related Art
In an active-matrix display device such as a liquid crystal display device or an organic electroluminescence (EL) display device, a thin-film transistor called the TFT is used. In such a display device, thin-film transistors are arranged in an array to constitute a thin-film transistor array device.
Generally, in each thin-film transistor, a semiconductor layer which is made of silicon or the like and is to be a channel portion is formed as an amorphous or crystalline semiconductor film; however, as the semiconductor layer that is to be the channel portion, it is preferable to use the crystalline semiconductor film which has higher mobility than the amorphous semiconductor film. Generally, the crystalline semiconductor film is formed by forming, and then crystallizing, the amorphous semiconductor film.
The method of forming the crystalline semiconductor film by crystallizing the amorphous semiconductor film includes: an excimer laser annealing crystallization (ELA) method; a thermal annealing crystallization method using a Ni catalyst or the like; and a crystallization method using a combination of an infrared semiconductor laser beam and a sample structure having a light absorbing layer.
However, the general crystallization using the ELA method forms a crystalline semiconductor film including microcrystals or polycrystals, thus causing variation in electrical characteristics depending on the size and distribution of crystal grains (crystalline structure). Use of such a crystalline semiconductor film in the TFT causes variation in the characteristics of the TFT.
On the other hand, the thermal annealing method allows uniform crystallization, but processing of a catalyst metal is difficult. In addition, the crystallization method using the combination of the infrared semiconductor laser beam and the sample structure having a light absorbing layer requires a process of forming, and then removing, a light-absorbing layer and a buffer layer on the sample, and this indicates a problem in terms of takt time. Furthermore, there is another problem that the TFT, which is manufactured using the film crystallized by the solid-phase growth method, falls short of targeted electrical characteristics due to a small average grain size of the film.
Thus, in the ELA method, a conventionally suggested technique is to allow control of a crystal grain width in the TFT crystalline semiconductor film (Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2008-85317). In addition, in the ELA method, another suggested technique is to allow control of the direction of the crystalline grain boundary or crystalline grain width in the TFT crystalline semiconductor film (Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2008-85318).
When using the techniques disclosed by Patent References 1 and 2, it is possible to form a crystalline semiconductor film including crystals having a large grain size of 0.5 μm to 10 μm. In addition, by forming a semiconductor element using the film thus formed, it is possible to manufacture a satisfactory semiconductor device with reduced variation between adjacent elements.